1. Field of the Invention
The present invention relates to a fabrication method for an integrated circuit device. More particularly, the present invention relates to a fabrication method for a two-bit flash memory cell.
2. Description of the Related Art
FIG. 1 is a schematic, cross-sectional view of an EPROM tunnel oxide (ETOX) structure of a flash memory cell according to the prior art. The ETOX structure is basically the addition of a tunnel oxide layer to the traditional erasable and read-only memory cell. As shown in FIG. 1, beside an oxide layer not being shown in the Figure, the conventional flash memory cell, as illustrated in FIG. 1, comprises a polysilicon gate which is the control gate 104, a polysilicon layer which is the floating gate 102, a source region 106a and a drain region 106b, wherein the source/drain regions 106a, 106b are formed near the two sides of the floating gate 102 in the substrate 100.
The method for programming a bit of binary data to a flash memory cell is to apply a positive voltage to the control gate and a lesser positive voltage to the drain region 106b (or source region 106a) with the source region 106a (or drain region 106b) being grounded. Hot electrons generated between the drain region 106b (or source region 106a) and the substrate are injected into and stored in the floating gate 102. The conventional flash memory cell, however, can only store a data of "1" and "0", and is thereby known as the single-bit flash memory cell.
A drawback of this type of single-bit flash memory cell is that after the hot electrons are injected into the floating gate 102, they do not stay at the two ends of the floating gate 102. The hot electrons tend to redistribute themselves throughout the entire floating gate 102. The redistribution occurs regardless whether that the hot electrons are injected from the source region 106a or from the drain region 106b into the floating gate 102. As a result, the operating mode of this type flash memory cell is limited to a single-bit storage. The occurrence of redistribution not only poses difficulties in the reading operation, it also leads to the serious problem of an over-erase.